Re: [RFC PATCH v1 09/28] mm: abstract shadow stack vma behind `arch_is_shadow_stack`

[Deepak Gupta]

On Thu, Jan 25, 2024 at 09:18:07AM +0100, David Hildenbrand wrote:
> On 25.01.24 07:21, debug@xxxxxxxxxxxx wrote:
> > From: Deepak Gupta <debug@xxxxxxxxxxxx>
> >
> > x86 has used VM_SHADOW_STACK (alias to VM_HIGH_ARCH_5) to encode shadow
> > stack VMA. VM_SHADOW_STACK is thus not possible on 32bit. Some arches may
> > need a way to encode shadow stack on 32bit and 64bit both and they may
> > encode this information differently in VMAs.
> >
> > This patch changes checks of VM_SHADOW_STACK flag in generic code to call
> > to a function `arch_is_shadow_stack` which will return true if arch
> > supports shadow stack and vma is shadow stack else stub returns false.
> >
> > There was a suggestion to name it as `vma_is_shadow_stack`. I preferred to
> > keep `arch` prefix in there because it's each arch specific.
> >
> > Signed-off-by: Deepak Gupta <debug@xxxxxxxxxxxx>
> > ---
> >  include/linux/mm.h | 18 +++++++++++++++++-
> >  mm/gup.c           |  5 +++--
> >  mm/internal.h      |  2 +-
> >  3 files changed, 21 insertions(+), 4 deletions(-)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index dfe0e8118669..15c70fc677a3 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -352,6 +352,10 @@ extern unsigned int kobjsize(const void *objp);
> >   * for more details on the guard size.
> >   */
> >  # define VM_SHADOW_STACK	VM_HIGH_ARCH_5
> > +static inline bool arch_is_shadow_stack(vm_flags_t vm_flags)
> > +{
> > +	return (vm_flags & VM_SHADOW_STACK);
> > +}
> >  #endif
> >  #ifdef CONFIG_RISCV_USER_CFI
> > @@ -362,10 +366,22 @@ extern unsigned int kobjsize(const void *objp);
> >   * with VM_SHARED.
> >   */
> >  #define VM_SHADOW_STACK	VM_WRITE
> > +
> > +static inline bool arch_is_shadow_stack(vm_flags_t vm_flags)
> > +{
> > +	return ((vm_flags & (VM_WRITE | VM_READ | VM_EXEC)) == VM_WRITE);
> > +}
> > +
>
> Please no such hacks just to work around the 32bit vmflags limitation.

As I said in another response. Noted.
And if there're no takers for 32bit on riscv (which highly likely is the case)
This will go away in next version of patchsets.

> --
> Cheers,
>
> David / dhildenb